1. Field of the Invention
The present invention relates to a semiconductor device for an IC tag, an IC tag, and a control method for an IC tag for detecting a command from radio waves transmitted from a reader/writer, and executing it.
2. Description of the Related Art
Recently in physical distribution management at factories and in stock control at retail stores, RFID (Radio Frequency Identification) technology is receiving attention. This is a technology for attaching a tag having an IC, in which the specific information of a product is written, to the product and reading the information via radio antenna.
In this IC tag for RFID (hereafter called IC tag), an IC chip and an antenna are integrated. The IC chip has a memory for storing data. The antenna is installed for receiving radio waves. The IC tag receives RF signals from a reader/writer and performs the operation according to the RF signal. There are two types of IC tags: an active type which comprises a battery, and a passive type which does not comprise a battery. The passive type IC tag receives power from the reader/writer via the RF signals, and secures power for operating the IC chip by this (see Udo Karthaus, et al. “Fully Integrated Passive UHF RFID Transponder IC With 16.7 μW minimum RF input Power”, IEEE Journal of Solid State Circuits, Vol. 38, NO. 10, October 2003, pp. 1602-1608). In other words, radio communication by RF signals is performed between the IC tag and the reader/writer, and these RF signals are used for supplying power to the IC tag.
The IC tags are classified into an IC tag for having the enclosed IC chip execute various functions to achieve multi-functions, high performance and high reliability, and an IC tag for simply storing identification numbers and simple information to the enclosed IC chip, depending on the purpose of use. Particularly in the case of the latter IC tag, the major purpose is providing an inexpensive IC tag without having unnecessary functions, and minimizing the area of the enclosed IC chip. The passive type IC tag, which does not enclose a power supply, is a typical example thereof.
As described above, the IC tag acquires a command comprised only of an instruction command, or a command comprised of an instruction command and data by radio communication using RF signals (modulated waves) after modulating the carrier, which is output from the reader/writer. The modulation method is, for example, ASK (Amplitude Shift Keying), PSK (Phase Shift Keying) or FSK (Frequency Shift Keying).
The IC tag receives modulated waves by an antenna. An antenna is roughly classified into an antenna for radiation electromagnetic waves (frequency band primarily used is 2.45 GHz), such as a dipole antenna, and an antenna for induction electromagnetic waves (frequency band primarily used is 13.56 MHz), such as an antenna coil. When the IC tag receives the modulated waves by the antenna, the receive unit of the IC chip demodulates the received data into binary data (digital data) of “0” and “1”, according to the modulation method being used. The demodulated digital data is sent to the signal processing unit in the IC chip, where the command is analyzed, and such processing as access to internal memory is performed.
However during the execution of the processing based on the received command in the IC tag, a modulated wave including the frame pulse is being transmitted from the reader/writer. At this time the IC tag continues executing the current processing if only the frame pulse is included in the modulated wave, but if noise is included in the modulated wave other than the frame pulse, and should the IC tag detect this noise as data, it is judged as the arrival of a new command, and the currently executing processing is stopped. In this case, the processing based on the received command cannot be ended normally because of the noise.